Calibration device for extruded sections

ABSTRACT

A calibration device for extruded sections comprises at least two calibration plates ( 10 ) parallel to one another at a distance apart such as to form a gap ( 1 ) through which an extruded section ( 11 ) can pass, each calibration plate comprising cooling means for cooling the plate and vacuum-forming means for creating a vacuum in the gap ( 1 ), the calibration device comprising air-intake means for introducing air into the gap ( 1 ), said means being designed to form air cushions or pads between the surfaces of the extruded section ( 11 ) and the surfaces of the calibration plates ( 10 ).

[0001] The present invention relates to a calibration device forextruded sections, in particular for hollow sections, pipes and alveolarplates of any geometrical structure and made of any extrudablethermoplastic material.

[0002] In systems for extrusion of plastic materials, molten extrudedplastic material coming from an extrusion die head and having aclosed-cell geometrical shape is made to pass through a calibrationassembly which enables the surfaces of the extruded section to be cooledoff, at the same time maintaining the geometrical shape impressed by thedie head.

[0003] The calibration assemblies according to the prior art areprovided with calibration plates as illustrated in the cross-sectionalview of FIG. 1 and designated by the reference number 100. FIG. 1 showstwo calibration plates 100 arranged at an appropriate distance apart soas to leave a gap 101 through which the extruded section can pass.

[0004] Each plate 100 is provided with ducts 102 for the circulation ofa coolant and ducts 103 for vacuum. The ducts 103 for vacuum have radialslits 104 that set them in communication with the gap 101 through whichthe extruded section is to pass. The cooling ducts 102 are connected toa cooling system, and the vacuum ducts 103 are connected to avacuum-forming system.

[0005] In order to enable cooling and maintenance of the geometricalshape of the extruded section, the calibration plates 100 are providedwith a cooling circuit, made up of the ducts 102, to which arefrigerating unit may be connected in the case where an alveolarsection of plastic material of the PP-HDPE type is to be produced, orelse a thermoregulation control unit may be connected in the case wherethe extrusion material employed is PC-PMMA.

[0006] In order to cause the surfaces of the extruded section to adhereor closely fit to the calibration plates 100 and then performcalibration of the extruded section, vacuum pumps are used which areconnected to the vacuum ducts 103, in such a way as to create a vacuumin the slits 104 and hence in the gap 101, thus causing the surface ofthe extruded section to adhere to the plates 100.

[0007] However, the aforesaid calibration devices according to the priorart have a number of drawbacks.

[0008] In fact, the adhesion of the extruded section to the surfaces ofthe calibration plates 100 due to vacuum inevitably creates aconsiderable friction between the extruded section and the plates. Thisimpairs the ease of slip of the extruded section, and consequentlyreduces the sliding speed of the extruded section, and hence the out putcapacity of the system or plant.

[0009] In addition, the aforesaid high-friction sliding of the extrudedsection on the calibration plates spoils the surface quality of theproduct and decreases its optical characteristics. For this reason, withthe prior art calibration devices it is not possible to obtain perfectlysmooth surfaces having optimal optical characteristics.

[0010] An object of the present invention is to eliminate the drawbacksof the prior art by providing a calibration device for extruded sectionsthat enables high production rates to be achieved and at the same timeensures an excellent surface quality of the extruded section.

[0011] Another object of the present invention is to provide such acalibration device that is economical and simple to make.

[0012] The above objects are achieved in accordance with the inventionthanks to the characteristics listed in the attached independent claim1.

[0013] Advantageous embodiments of the invention appear in the dependentclaims.

[0014] The calibration device according to the invention comprises atleast two calibration plates arranged parallel to one another and at adistance apart such as to form a gap through which an extruded sectioncan pass. Each calibration plate comprises a cooling circuit for coolingthe plate and a vacuum circuit for creating a vacuum in the gap in whichthe extruded section is to pass.

[0015] The peculiar feature of the calibration device according to theinvention is that the said device is provided with air-intake means forintroducing air into the gap between the calibration plates in such away as to form air cushions or pads between the surfaces of the extrudedsection and the surfaces of the calibration plates.

[0016] The said air cushions tend to reduce the sliding friction betweenthe surfaces of the section that advances through the gap and thesurfaces of the calibration plates. In this way, the section can advanceat a higher speed than in the case where no air cushions are provided,with the result that a higher production rate is achieved.

[0017] In addition, the aforesaid reduction in the sliding frictionbetween the surfaces of the section and those of the plates guarantees alower degree of deterioration of the surfaces of the section, with theresult that products with a better surface finish, and hence a betteroptical quality, are obtained.

[0018] In addition, the air introduced into the gap of the calibrationdevice is sucked in by means of the vacuum circuit. Consequently, thereis a continuous circulation of fresh, cool air which comes into contactwith the surfaces of the section, thus improving the coolingcharacteristics of the calibration device according to the invention.

[0019] Further characteristics of the invention will emerge more clearlyfrom the ensuing detailed description relating to an embodiment thereofwhich is presented purely by way of example and hence in no way limitsthe scope of the present invention, and which is illustrated in theannexed drawings, where:

[0020]FIG. 1 is a longitudinal sectional view of a calibration deviceaccording to the prior art;

[0021]FIG. 2 is a longitudinal sectional view of a calibration deviceaccording to the present invention; and

[0022]FIG. 3 is cross-sectional view, taken along the line III-III ofFIG. 2, illustrating an alveolar section enclosed between twocalibration plates.

[0023] With reference to FIGS. 2 and 3, a calibration device, designatedas a whole by reference number 20, according to the invention isdescribed. The calibration device 20 comprises two calibration plates10, each having a substantially rectangular cross section andlongitudinal section. The two calibration plates 10 are set parallel toone another at a distance apart such as to form a gap 1 through whichthe extruded section is to pass. By way of example, FIG. 3 shows anextruded section in the form of an alveolar section 11.

[0024] Each calibration plate 10 comprises a plurality of cooling ducts2, which are connected together and form the cooling circuit of thecalibration device 20. The cooling ducts 2 can be set in communicationwith a refrigerating system which introduces coolant into the ducts 2.

[0025] Each calibration plate 10 moreover comprises a plurality ofvacuum ducts 3 which are connected together and constitute a circuit forforming a vacuum in the gap 1 of the calibration device 20. The vacuumducts 3 can be set in communication with a pneumatic system, such as avacuum pump, which sucks air from the said vacuum ducts 3.

[0026] Each vacuum duct 3 is in communication with a respective vacuumslit 4 which opens into the gap 1. In this way, the gap 1 is incommunication with the vacuum ducts 3 through the vacuum slit 4.Consequently, when the pneumatic system sucks in air from the vacuumducts 3 through the vacuum slits 4 a negative pressure is created in thegap 1, which enables the surfaces of the section 11 to closely fit tothe surfaces of the calibration plates 10.

[0027] According to the invention, each calibration plate 10 moreovercomprises a plurality of air ducts 5, connected together andconstituting a circuit for air intake into the calibration device 20.The ducts 5 for air intake can be set in communication with thepneumatic system, which introduces compressed air into them.

[0028] Each air duct 5 is in communication with a respective air slit 6which opens out into the gap 1. In this way, the gap 1 is incommunication with the air ducts 5 through the air slit 6. Hence, whenthe pneumatic system introduces compressed air into the air ducts 5,through the air slits 6, air is introduced into the gap 1 against thesurfaces of the section 11.

[0029] In this way, air cushions are created between the surfaces of theextruded section 11 and the calibration plates 10. The said air cushionsenable a reduction in the friction generated between the surfaces of thesection 11 and the plates 10.

[0030] The pressure and rate of supply of the air cushions can beregulated. In fact, after coming into contact with the surfaces of theextruded section within the gap 1, air is sucked through the vacuumslits 4 in a continuous process. This continuous process of introductionand suction of air means that new cool air in contact with the surfacesof the section 11 is always available, and at the same time the coolingcapacity of the calibration device 10 is increased.

[0031] Numerous modifications and variations as regards details, all ofwhich lying within the reach of a person skilled in the art, may be madeto the present embodiment of the invention, the said modifications andvariations falling in any case within the scope of the present inventionas expressed in the appended claims.

1. A calibration device for extruded section comprising at least twocalibration plates (10) set parallel to one another at a distance apartsuch as to form a gap (1) through which an extruded section (11) canpass, each calibration plate comprising cooling means for cooling theplate and vacuum means for creating a vacuum in said gap (1),characterized in that said calibration device comprises air-intake meansfor introducing air into said gap (1), said means being designed to formair cushions between the surfaces of the extruded section (11) and thesurfaces of the calibration plates (10).
 2. The device according toclaim 1, characterized in that said air-intake means comprise an aircircuit made in said calibration plates (10) and connected to apneumatic system.
 3. The device according to claim 2, characterized inthat said air circuit comprises a plurality of air ducts (5)communicating together, and air slits (6) that set said air ducts (5) incommunication with said gap (1).
 4. The device according to any one ofthe preceding claims, characterized in that control means are providedfor controlling the pressure of air introduced by said air-intake means.5. The device according to any one of the preceding claims,characterized in that said vacuum means comprise a vacuum circuit madein said calibration plates and connected to a pneumatic systemcomprising a vacuum pump.
 6. The device according to claim 5,characterized in that said vacuum circuit comprises a plurality ofvacuum ducts (3) which are provided in said calibration plates, andvacuum slits (4) that set said vacuum ducts (3) in communication withsaid gap (1).
 7. The device according to any one of the precedingclaims, characterized in that said cooling means comprise a coolingcircuit made in said calibration plates and connected to a coolingsystem.
 8. The device according to claim 7, characterized in that saidcooling system comprises a plurality of cooling ducts (2) which areprovided in said calibration plates and through which a coolant canflow.